Bunch Lengthening in SOR

Bunch lengthening in SOR, a 400 MeV electron storage ring dedicated for synchrotron radiation experiments, was found to be induced by a longitudinal coupled bunch instability. The bunch length which increases as a 1/5.8 power of the beam current, can be explained by the balance between the growth of the instability and Landau damping due to the spread of synchrotron frequency, and by introducing a weak frequency dependence of coupling impedance. In connection with this phenomenon, slow fluctuation of synchrotron radiation was found to come from a detailed unbalance of the equilibrium electron distribution on the longitudinal phase space. Touschek life time of the expanded beam and Landau cavity to suppress the instability are described.